Electric double-layer capacitors and lithium ion capacitors are used as small power sources for semiconductor memory backup and the like. These capacitors, however, have problems such that in cases where polarizing electrodes made from cheap porous carbon materials such as activated carbon are used, application of such a high voltage that exceeds the decomposition voltage of an electrolyte solution may cause an increase in internal resistance or a decrease in electrostatic capacity.
Therefore, electrolyte solutions used for electric double-layer capacitors or lithium ion capacitors are required to have high conductivity and high electrochemical stability. In addition, it is important for electrolyte solutions used for these capacitors, which are possibly used under unforgiving conditions, to have characteristics of keeping the capacitors stably active for a long time in a wide temperature range of from low temperatures to high temperatures.
Patent Literature 1 discloses an electrolyte solution for electric double-layer capacitors prepared by dissolving tetraethylammonium tetrafluoroborate, which is an aliphatic quaternary ammonium salt, as an electrolyte in propylene carbonate as an organic solvent.
However, in the electrolyte solution containing propylene carbonate as a solvent, the solvent begins to decompose at an applied voltage of 2.6 to 2.8 V. Therefore, the maximum voltage to be applied to an electric double-layer capacitor using this electrolyte solution is around 2.5 V. Thus, such a capacitor is problematically insufficient in withstand voltage.
Patent Literature 2 discloses an electrolyte solution using a mixture of sulfolane and 3-methylsulfolane or a mixture of sulfolane and 2,4-dimethylsulfolane as an organic solvent in order to increase a withstand voltage.
However, sulfolane has a melting point of 29° C., 3-methylsulfolane has a melting point of 1° C., and 2,4-dimethylsulfolane has a melting point of −3° C. Their melting points are relatively high. Therefore, the electrolyte solution coagulates at low temperatures to significantly deteriorate the characteristics of the capacitor.
Patent Literature 3 discloses the use of an electrolyte solution prepared by dissolving a quaternary onium salt in a solvent that contains 15 to 85% by volume of sulfolane or its derivative and 85 to 15% by volume of ethyl methyl carbonate as a chain carbonate ester, in an organic electrolyte cell including an electrode made of an organic semiconductor material having a polyacene skeleton.
This method, however, has a problem that ethyl methyl carbonate, which has poorer oxidation resistance than sulfolane, has remarkable influence to reduce the withstand voltage of the electrolyte solution.
Patent Literature 4 discloses the use of an electrolyte solution, in an electric double-layer capacitor, prepared by dissolving a quaternary ammonium salt such as spiro-(1,1′)-pyrrolidinium tetrafluoroborate in an organic solvent which is a mixture of sulfolane and a chain alkyl sulfone compound mixed in a ratio of 70:30 to 90:10.
However, this electrolyte solution may coagulate at extremely low temperatures, and is not satisfactory in view of long-term reliability.